1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device for improving the problems of response times with regard to a liquid crystal display.
2. Background of the Invention
Recently, a liquid crystal display (LCD) equipped with thin film transistors (TFT) has developed significantly due to its characteristics including light weight, thin shape and low power consumption. Conventionally, the use of LCDs for PCs was mainly directed to displaying static images, however, they have been substituted for CRTs such as when displaying moving pictures in a graphics system or when displaying video images on monitors, so that there is a growing concern about displaying moving pictures using LCDs.
While a CRT is in the impulse type of light emission, an LCD is in the hold type with emitting a continuous light during a whole period of a frame, thus being unable to follow the CRT in terms of a quality of moving pictures if leaving the LCD as it is. Accordingly, there have been proposed a scheme for doubling the refresh rate or the blanking scheme for emitting a light intermittently for each frame in order to obtain the similar characteristics to CRTs for moving pictures. This is an ideal solution but requires a special liquid crystal with a very high speed response, so that the liquid crystals currently in use are not applicable due to their slow response.
For example, a present TN mode TFT-LCD has its on/off response time of about 1 refresh cycle (16.7 ms at 60 Hz refresh), however, the response time delays greatly in a halftone level, resulting in up to a few to ten refreshes. In particular, video images of such as TVs or the like mostly have halftone images, so that correct brightness can not be obtained. Even when displaying text data on PCs, it takes a long time for a screen to become a good condition where one can easily read it when he or she performs a scroll operation.
As above, a deterioration in image quality when displaying moving pictures on a TFT-LCD results from the fact that a transition of brightness of each pixel does not complete within one frame period of 16.7 ms. Namely, even in the case of liquid crystals with a fast response, the capacitance of the liquid crystal changes based on the principle of driving of the liquid crystal, thus the targeted brightness can not be achieved with only one time of charge/discharge of TFT as long as using the normal driving method. Accordingly, the display response is unable to catch up with the image when it changes for each frame. Furthermore, since the response time differs between R(red), G(green) and B(blue) when displaying color images because the response time varies depending on gradations, a remarkable hue variation (color shift) may occur in boundary areas of moving edges or thin lines.
There exists a method called overdrive for resolving the delay of the response time. This method is to improve the response characteristics to a step input for the liquid crystal device by supplying a voltage greater than the targeted voltage at the first frame of input changes in order to accelerate a transition of brightness. For example, Japanese Unexamined Patent Publication No. 1995-12138 discloses a technique where operation timing of a time division light-emitting device of three primary colors (RGB) is delayed by an amount equivalent to the optical response time of the liquid crystal and light is not emitted for a period corresponding to the optical response time in order to implement a color reproduction and further the image signal amplitude is increased to compensate for inadequate writing for halftones.
As described above, for the LCDs with a slow response time, when telop opaque projector) or any daubed area with a sharp boundary is run, some color differing from the original one would be seen on the boundaries depending on the moving speed because the response time for halftones differs between R, G and B sub-pixels, thereby causing a color shift. Even if tolerating the boundary areas blurring due to the slow speed of gradation changes, the color on the boundary area ought to be a mixture of the previous and subsequent colors of that boundary. However, another hue differing from the essential color mixture might occur when the response time differs between R, G and B. A range where this color shift occurs would extend from the boundary to a point which will be reached for one frame period with the moving speed if the difference of response times between R, G and B sub-pixels for the gradation change settles within one frame period. However, if it takes n frame periods for settlement, the color shift would occur for n times of the number of pixels.
The overdrive technique allows matching the response time of each sub-pixel to about one frame period, however, it can not accelerate a transition to a full OFF state, i.e., 0V. When not allowed using a voltage which exceeds the voltage used for statically defined gradations (i.e., overvoltage range), there may occur a case where it is impossible to respond within one frame period in the on-direction transition. Furthermore, particularly seen in the TN mode liquid crystals, the effective brightness (average brightness) can not be matched within one frame period even by using the overdrive technique because the response time changes depending in particular on the starting gradation and targeted gradation.